Fume cabinets are usually used to isolate experiments or tests from the environment and from the experimenter. In particular, they are usually used to protect the experimenter from emissions produced by the test process, to protect the experiment or test from contamination by unwanted gases, particulates or bacteria, and to protect the environment from the products of the test process.
Conventional fume cabinets currently in use are generally based on the "counterflow" principle. In such cabinets the test or experiment is usually located in a space which is enclosed except for a large front opening to allow the experimenter access to the test or experiment. Air is drawn into the cabinet through the front opening, and the air flow into the cabinet is supposed to prevent contaminants in the cabinet from travelling outwardly through the front opening.
In such counterflow fume cabinets, the physical mechanisms available for transport of contaminant gasses outwardly through the front opening are molecular and turbulent diffusion. When the air flow into the front opening is strictly laminar, only molecular diffusion occurs, and calculations of molecular concentration show that it falls off rapidly with upstream distance. With a typical value of the binary diffusion coefficient, and an airflow into the cabinet front opening of about one metre per second, the contaminant concentration may typically decrease as much as six orders of magnitude in an upstream distance of only one millimeter. Thus, it is easy in an ideal laminar flow situation to ensure a negligible concentration upstream of the plane of the cabinet front opening (usually called the "face"). The net result is similar for particulates, although the physical mechanism for transport of particulates is quite different.
However the actual realization of the counterflow principle in practical fume cabinets is far from ideal. Typically there is a moveable sash at the top of the face which partially obstructs the entry; the exhaust from within the fume cabinet is from the top instead of from the back; the air exterior to the cabinet is not quiescent but normally is in motion; and the presence of an operator near the face, and of apparatus inside the working space, generate turbulent wakes which destroy the uniformity and laminarity of the flow.
In the design of the best fume cabinets, great care is taken, with a variety of flow control devices, to achieve a uniform inlet velocity at the face in the absence of an operator. The face velocity is the central feature in most fume cabinet specifications and is typically about 0.5 meters per second. With such fume cabinets very low contaminant concentrations are achieved in practice outside the face under ideal conditions However when conditions become non-ideal, e.g. in the presence of a turbulent wake produced by a manikin, the distance required between source and measurement point to achieve a reduction in concentration of six orders of magnitude is about 20 centimeters, as compared with 1 millimeter for ideal laminar flow.
An even more serious non-ideal condition is external air movement, which, if it exceeds 50 per cent of the face velocity, can drastically reduce the containment of the fume cabinet. Thus, cross flows at the face of the order of about 0.25 metres per second are too large to be tolerated by most conventional fume cabinets. However such speeds can commonly be produced by personnel traffic, ventilating flows, open doors and windows, and the like.
An entirely different approach to containment is the air curtain principle. In this concept, "face velocity" becomes irrelevant since containment is based on the property of the air curtain as a barrier to mass transport. So far as is known, there are currently no fume cabinets marketed using the air curtain principle. However a form of such fume cabinet was described in German Offenlegungrschrift 29 17 853 published Nov. 6, 1980. In this cabinet, a curtain of air is directed upwardly at the face opening, to prevent contaminants inside the cabinet from reaching the outside. As will be explained later in this description, the applicant has determined that the air flows used in the German document are insufficient to prevent spill-back of contaminated curtain air into the room at the top of the face opening.
As will be explained, certain minimum exhaust air flows are needed to provide reasonable assurance that the curtain will not spill back such contaminated air. The minimum flow needed is found, surprisingly, to be considerably more than that which might have been expected. However it is still less than that of many conventional counterflow fume cabinets, and it provides better resistance to crosswinds.
The use of an air curtain to protect an operator from harmful fumes while permitting the operator to have access to a working space was also described in British patent 1,582,438 published Jan. 7, 1981 to Imperial Chemical Industries Ltd. However in that patent, the air curtain together with noxious gases from the process are removed via a flue, and there is no indication of the flows required to prevent or reduce the likelihood of migration of contaminants through the curtain. As will be discussed, the ratio of exhaust to jet flows for a given range of curtain jet height to thickness ratio is important in order to improve the barrier properties of the curtain.